Vapor vacuum pump



July 16, 1946. P. ALEXANDER ET AL VAPOR VACUUM PUMP Filed May 26, 1945 C S6 IL \^//l AE'Y By n A orney Patente-d July 16, 1946 S PATENT OFFICE VAPOR VACUUM PUMP Paul Alexander, Berkhamsted, and Cecil Whiley,

London, England Application May 26, 1945, Serial No. 596,036 In Great Britain April 21, 1945 1 Claim. l

This invention relates to vapor vacuum pumps.

Mercury is the liquid usually vaporised in vapor pumps, but other liquids, such as oils, are employed. In the usual type of mercury vapor pump, mercury vapor from a boiler ascends a central tube and enters an annular jet at the top of the tube. Mercury vaporissues from the jet downwards into the annular space between the central tube and a water-cooled outer tube. The upper end of the outer tube is the inlet of the pump and the outlet is a pipe connecting the lower part of the annular space between the central and outer tubes to the backing pump. The mercury vapor from the jet is condensed by contact with the cooled outer tube and returns to the boiler.

The molecules of mercury vapor collide with the air molecules in the annular space driving them downwards, thus drawing air from the inlet and driving it towards the outlet. The eciency of the pump depends on the downward velocity of the molecules of the mercury vapor issuing from the jet as well as on the density of the vapor.

The object of the invention is to improve the eciency of the pump by maintaining this downward velocity of the molecules.

According to the invention, a water-.cooled jacket surrounds, but does not touch, the central tube over a length thereof from just below the jet orifice downwards.

In the accompanying drawing, which shows in a single gure by way of example a form of pump embodying the invention, in vertical section, the mercuryboiler I, containing mercury 2 with an electric heater in the space 3, supports the central vapor tube 4 and the outer tube '5, forming the body of the pump, which terminates above the inlet 6 of the pump. The annular jet 1, with annular orifice 8, is mounted on the upper end of the tube Il. The tube 4 may either extend right up to the annular jet as illustrated in co-pending application No. 596,035 or, as shown in the embodiment illustrated in the accompanying drawing, the tube 4 terminates below the annular jet, and the latter is fixed on a tube 9 adapted to slide over the upper end of the tube 4.

The Vapor from the mercury 2, passing up the tube 4, enters the jet 'I through holes I0 in the central body of the jet, and issues from the orifice 8 into the annular space Il between the tubes 4 and 5 in which the pumping action takes place, the pumping direction being 4indicated by the dotted line I2.

The jet is adjustable in height relatively to the tube 5 by means of the screw I3 engaging a nut I4 carried by a spider l5 xed to the tube 5.

CTI

The outlet of the pump is the pipe I6 connecting the space I l to the usual backing pump. The tube 5 is surrounded by a water jacket I1 to condense the mercury vapor which falls into the seal I8, and thence returns through holes I9 into the boiler l.

In operation the mercury vapor issuing from the orice 8 into the space II causes air to be withdrawn from the inlet 6 and is driven downwards by the vapor towards the outlet IS which is connected with a backing pump.

As described and claimed in co-pending application No. 596,034 the tube 5 diminishes in diameter by the inclusion of a conical wall just below the jet 1, which conical wall joins up with the parallel walls of the lower part of the tube 5, so that the tube 5 diminishes in diameter at a point near jet oriiice level so that the length of vapor jet measured along a line B with angle l5 degrees from the general pumping direction, indicated by the line I I, does not exceed 2.6 times the width A of the annular space at jet oriiice level.

The air pressure is a minimum in the annular space II just below the jet orice and from this point rises rapidly with increasing distance downwards from the jet orice, till it reaches a pressure above that in the outlet tube I6. The air at points of high pressure tends to ow backwards up the annular space and through the mercury vapor to the inlet E, and any air which does so flow backwards diminishes the eiiiciency of the pump.

Such backward flow of the air can be prevented only by ensuring that every air molecule is struck by downwardly directed molecules of mercury vapor and driven downwards. The efficiency of the pump therefore (assuming adequate density of the mercury vapor) depends on the downward velocity of its molecules, and the Velocity is particularly important in the annular space from the neighborhood of the reference Figure 11 downwards and past the portion of the outer tube 5 which is shown conical in the drawing, because it is over this length of space that the gradient of the air pressure is most steep.

It has been found that the eiciency of the pump is increased by surrounding that portion of the central tube 4 which passes through this length of space by a Water jacket 25, supplied with water by the pipes 26. The water jacket is out of contact with the tube ll, since any appreciable cooling of the tube is disadvantageous. The reason for the resulting increase in efficiency is that, without the jacket 25, vapor molecules which hit the hot wall of the tube lose their downward velocity and rebound with random velocity, equally distributed in al1 directions. These rebounding molecules collide with the molecules of the vapor issuing from the jet and diminish the downward component of their velocity. Since this is the component effective for pumping, the pumping eciency is diminished as the result of molecules striking the not wall of the tube 4.

When the molecules are prevented by the water jacket 25 from striking the hot wall, this harmful eiect is avoided, and the eiciency of the pump is increased.

The extent of the tube 4 surrounded by the Water jacket may be increased beyond that shown in the drawing, but the advantage gained is small,

while some disadvantage results if the vapor 15 i ascending the tube 4 to the jet 1 is appreciably cooled.

We claim:

A vapor vacuum pump comprising a central tube and an outer tube surrounding the same and forming therebetween an annular space, an annular jet on said central tube having a jet orice arranged to project vapor therefrom downwardly into said annular space, and a watercooled jacket surrounding and spaced from said central tube and extending downwardly over a length thereof from just below said jet orice.

PAUL ALEXANDER. CECIL WHILEY. 

